1. Field of the Invention
The present invention relates to a diesel engine fuel injection control device and a fuel injection control method that carries out a small pilot injection before a main injection of fuel.
2. Description of the Related Art
With conventionally known technology, in a diesel engine, a small pilot injection is carried out in advance of the main injection of fuel followed by the suppressing combustion noise and the generation of NOx.
In the meantime, in a diesel engine, the cylinder temperature at the end of compression stroke is low when the engine is cold such as during low temperature starts making it easy for white exhaust to occur that contains fuel which has not yet undergone combustion as the main component. Because of this, when the water temperature (equivalent to engine temperature) is low in a device disclosed in Japanese Patent Application Kokai (Laid-Open Publication) No. 1-155053, in order to suppress the generation of noise, NOx and white smoke (gas which has not yet undergone full combustion), the pilot injection timing is advanced with the main injection timing remaining fixed without change and an increased portion of the ignition delay of the pilot injection fuel is compensated for.
If the pilot injection timing is advanced, it is assumed that the generation of white smoke will be suppressed due to the following reasons. Namely, by carrying out an advance, it is assumed that the peak of heat generation rate due to combustion will almost match top dead center (TDC) and the temperature inside a cylinder will increase up to a maximum temperature achievable in this state and in addition, because the engine will operate at maximum efficiency, the quantity of fuel that is used will be at a minimum and the portion of the fuel that will not undergo combustion will correspondingly be reduced.
However, the above-mentioned conventional device also has problems for the following reasons.
Namely, in this fuel injection control apparatus for a diesel engine, control is carried out by determining the amount the pilot injection timing is advanced based on the engine's cooling water temperature only and then specifically increasing the amount of advance with the low temperature.
The inventors measured the density of white smoke within the exhaust gas with a PHS smoke meter under the following conditions: (1) when the water temperature was 0.degree. C. and the outside air temperature was 0.degree. C., and (2) when the water temperature was 0.degree. C. and the outside air temperature was -10.degree. C. The result of this experiment indicated that the density of the white smoke was less for (2).
The condition (1) represents a case where the water temperature and the outside air temperature are the same, i.e., when the engine was just started, and the condition (2) represents a case where the water temperature is higher than the outside air temperature, i.e., during engine warmup. From this it has been concluded that the latter had a smaller occurrence of white smoke with the same water temperature. This conclusion is considered to be due to the fact that the temperature inside an engine cylinder is higher during engine warmup with the same water temperature compared to directly after an engine start because the water temperature increases following increases in the temperature inside a cylinder leading to a state in which combustion can occur easily.
In the above-mentioned conventional arrangement, because the amount of advance is determined from the water temperature only, the amount of advance is identical for both (1) and (2). Therefore, if the amount of advance is set to conform to the state in (1), i.e., if the amount of advance is determined to suit a "directly-after-engine-start" situation, the value of advancement will not be suitable during engine warmup. In contrast, if the amount of advance is set to conform to the state in (2), i.e., if the amount of advance is determined to suit to a "during-engine-warmup" condition, the value of advancement will then not be suitable to directly after an engine start. For the former, pilot fuel is completely combusted before the compressive top dead center during engine warmup worsening the fuel consumption because of pumping loss during compression. For the latter, the ignition is delayed and the effect of suppressing the white smoke directly after an engine start is lessened. Because differences in the temperature inside a cylinder could not be considered in this type of conventional fuel injection control apparatus without distinguishing the case directly after an engine start and the case during engine warmup, it does not always follow that sufficient control can be carried out.